Highly Accurate Coupled Cluster Potential Energy Curves for the Benzene Dimer:  Sandwich, T-Shaped, and Parallel-Displaced Configurations

State-of-the-art electronic structure theory has been applied to generate potential energy curves for the sandwich, T-shaped, and parallel-displaced configurations of the simplest prototype of aromatic π−π interactions, the benzene dimer. Results were obtained using second-order Møller−Plesset perturbation theory (MP2) and coupled-cluster with singles, doubles, and perturbative triples CCSD(T) with different augmented, correlation-consistent basis sets. At the MP2 level, the smallest basis set used (a modified aug-cc-pVDZ basis) underestimates the binding by ∼0.5 kcal mol-1 at equilibrium and by ∼1 kcal mol-1 at smaller intermonomer distances compared to results with a modified aug-cc-pVQZ basis (denoted aug-cc-pVQZ*). The best MP2 binding energies differ from the more accurate CCSD(T) values by up to 2.0 kcal mol-1 at equilibrium and by more than 2.5 kcal mol-1 at smaller intermonomer distances, highlighting the importance of going beyond MP2 to achieve higher accuracy in binding energies. Symmetry adapted perturbation theory is used to analyze interaction energies in terms of electrostatic, dispersion, induction, and exchange-repulsion contributions. The high-quality estimates of the CCSD(T)/aug-cc-pVQZ* potential energy curves for the benzene dimer presented here provide a better understanding of how the strength of π−π interactions varies with distance and orientation of the rings and will assist in the development of approximate methods capable of modeling weakly bound π−π systems.